1. Field of Invention
The present invention relates to a field of USB communication, and more particularly to a circuit and method for producing USB host working clock.
2. Description of Related Arts
USB is the abbreviation of Universal Serial BUS. Because the communication speed of USB is fast, the interface of USB is simple, the application of USB is convenient, and other advantages of USB; USB has already become one of essential standard interfaces for PC, MP4, mobile phones, PDA (Personal Digital Assistant), digital cameras, printers, scanners, and other electronic equipments, and thus is applied widely in some aspects of information communication and data transmission.
Each of the conventional communication system needs a relatively accurate clock source. Using the clock source, a master clock that is needed by the communication system working is produced through the logic of the frequency division or the frequency multiplication inside the communication system, and the transmitted data flow is analyzed and collected, for achieving the purpose of data communication. The USB communication system is also without exception. In the high speed communication of USB (i.e., the communication speed reaches to 480 MHz), the USB communication system requires the higher accuracy of the data transfer clock (±0.5‰). Therefore, the USB communication system usually utilizes the clock of the external oscillator on the USB main structure. That is to say, the external part produces an accuracy clock (for example, 12 MHz) through the crystal oscillator, which is input to the internal part of the chip of the USB main structure and then is processed with frequency multiplication through the logical module of PLL, for producing the high speed working clock of 480 MHz that is finally needed by the system.
Because of the crystal oscillator, the chip of the USB main structure needs to configure at least two pins to the crystal oscillator. In that way, in the electronic products which have relatively less pins, the USB communication system can not be used to communicate. For example, in the conventional electronic products such as SIM card, pins are usually 4-7. Because of the demand of other system functions, no two extra pins can be configured to the crystal oscillator, so the USB main structure can not be used to communicate.
Moreover, along with the development of the manufacturing technique and the improvement of the design technology, the volume of electronic products is becoming smaller and smaller, and pins of electronic products are becoming fewer and fewer. However, the volume of the crystal oscillator is relatively big to the SOC (System on Chip), and thus the development of the high integration and the miniaturization of electronic products is restricted. Therefore, the crystal oscillator is the key factor to restrict the application and the development of the chip of the USB main structure.
Certainly, the internal part of the chip of the USB main structure is also able to generate the clock through the RC oscillator or the LC oscillator, and is provided to the system to use. Because of the technological error of the RC oscillator or the LC oscillator and the effect of other factors, the clock is generated by the oscillator in the internal part of the chip usually exits the error of ±20% with respect to the design objective, which is not able to satisfy the transmission accuracy requirement of the system.
Therefore, it is necessary to provide an advanced clock producing circuit and the method thereof, for the USB communication system to normally communicate, so as to cover flaws which have been mentioned above.